The present application relates to ultrasonic transducers.
Ultrasonic transducers are used for a wide variety of applications including imaging, nondestructive testing and heating. For example, in certain medical procedures ultrasonic energy is applied to heat tissue within the body of a living subject. In an ablation procedure, the tissue is heated sufficiently to kill undesired tissue as, for example, to about 60-80 degrees C. In ablation and similar procedures, it is highly desirable to heat the undesired tissue rapidly, so as to minimize collateral damage to neighboring tissue. Certain ablation techniques use ultrasonic transducers which are inserted into the body as, for example, on a catheter. Such transducers must be compact, but should be capable of emitting substantial ultrasonic power.
A typical ultrasonic transducer includes an active element such as a piezoelectric or magnetostrictive element. The active element physically deforms in response to an applied drive signal, most commonly an electrical signal. In operation, the active element is driven by a signal at an ultrasonic driving frequency and produces ultrasonic vibrations. The transducer may consist only of the active element, but typically includes additional structural elements. The active element and these additional structural elements are arranged to form a composite structure which resonates at the driving frequency. The vibrations from this structure are emitted into the surrounding medium. For example, in a typical medical application, the ultrasonic vibrations are emitted from the transducer into a liquid or gel medium and are transmitted through this medium into the tissue.
Typically, the transducer is arranged to emit acoustic vibrations from a front surface into the surrounding medium. The transducer typically has one or more acoustically reflective interfaces remote from the front surface, and typically to the rear of the active element. The interface or interfaces help to direct the ultrasonic energy out of the transducer through the front surface. As further discussed below, the term xe2x80x9cbacking interfacexe2x80x9d is used to refer to an interface which plays a significant part in the operation of the transducer. The resonant unit includes the structure between the backing interface furthest from the front or emitting surface and the emitting surface.
Some transducers employ a solid backing element having acoustic impedance different from the acoustic impedance of the active element. For example, a transducer incorporating a polymeric piezoelectric active element may include a solid backing element formed from a metal or ceramic. The interface between the backing element and the polymeric active element serves as a backing interface.
Other transducers, referred to as xe2x80x9cair-backedxe2x80x9d transducers, have a structure which provides an air layer at the rear surface of the active element. For example, as shown in U.S. Pat. No. 5,620,479, the interior bore of a tubular ceramic element is filled with air. The interface between the air and the element is highly reflective, because air has an acoustic impedance far lower than that of the ceramic. This interface serves as a backing interface, and helps to direct acoustic vibrations through the outer surface of the tubular element, which serves as the front or emitting surface of the transducer.
Air-backed transducers can provide good efficiency and can be compact. However, the emitting power of such a transducer is limited by thermal considerations. Air and other gasses provide only a limited cooling effect at the rear surface of the active element. The power of the applied drive signal must be limited to avoid overheating the transducer. This problem is particularly severe in the case of small transducers for applications such as ablation.
Thus, despite the considerable effort applied heretofore in development of ultrasonic transducers, further improvement is needed.
The present invention addresses these needs.
One aspect of the present invention provides an ultrasonic transducer. The transducer in accordance with this aspect of the invention includes a resonant unit which in turn includes an active element having a front surface facing in a forward direction and having a rear surface facing in a rearward direction. The active element may be a piezoelectric element or other element operative to generate ultrasonic vibrations in response to an applied signal. The resonant unit includes a liquid disposed to the rear of said active element. For example, the liquid may be in contact with the rear surface of the active element. The resonant unit is resonant at an ultrasonic frequency and adapted to emit ultrasonic vibrations principally in said forward direction.
A further aspect of the invention provides an ultrasonic transducer including an active element having front and rear surfaces. Here again, the active element is operative to generate ultrasonic vibrations in response to an applied signal. The transducer further includes a rear structure defining a space disposed to the rear of said active element. A liquid is disposed in this space. The element, rear structure and liquid cooperatively form a resonant unit having a backing interface. The liquid is disposed between the backing interface and the rear surface of the active element. In a particularly preferred arrangement, the rear structure may include a wall having a front surface facing toward the space and a rear surface facing away from the space. A medium such as a gas having acoustic impedance lower than the acoustic impedance of the liquid abuts the rear surface of the wall to form a backing interface.
A transducer according to a further aspect of the invention includes an active element, rear structure and liquid as discussed above. In this aspect of the invention, the liquid partially defines a backing interface of the transducer. For example, the rear structure may include a solid wall to the rear of the space, the wall having acoustic impedance differing from the acoustic impedance of the liquid, so that the interface between the liquid and the wall serves as a backing interface.
Preferred transducers in accordance with these aspects of the present invention are compact and efficient. However, because the liquid ss disposed within the resonant unit it can provide efficient cooling for the active element. The transducer desirably includes or is connected to a source of liquid arranged to move the liquid through the space. The most preferred transducers according to these aspects of the invention can provide output power higher than the power provided by an air-backed transducer of comparable size. Merely by way of example, a cylindrical, tubular transducer in accordance with one preferred embodiment is less than 3 mm in diameter but can provide over 50 Watts of continuous acoustic output power at about 9 MHz when operated in water. The preferred transducer according to the foregoing aspects of the invention can be use in various applications. For example, such transducers are especially valuable in ultrasonic ablation devices for insertion into the body of a subject.
Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.